1.0 Field of the Invention
The present invention relates to a RF (Radio Frequency) synthesizer and, more particularly, to an RF synthesizer having a correlator circuit that provides long term frequency stability for the RF synthesizer.
2.0 Description Related to the Prior Art
There are many types of RF (Radio Frequency) synthesizers in present use. These fall into one of four general categories: Phase Locked, YIG tuned, Direct Digital, and Digitally Tuned Oscillator. The phase locked synthesizer usually employs a Voltage Controlled Oscillator (VCO) that is phase locked to some fixed low frequency reference oscillator via a frequency divider circuit. The output frequency of the phase locked synthesizer is adjusted by changing the divider circuit characteristics. The phase locked synthesizer design is usually complex, as the frequency divider circuits are required to divide by non-integer ratios, and exhibits the frequency stability of the fixed low frequency reference, and, because of the narrow video bandwidths of the phase lock circuit, the phase locked synthesizers tend to be slow in its response (1 ms to 10 ms, typically).
The YIG (Yittrium Iron Garnet) tuned synthesizer provides a wide band (usually up to an octave relative to the input frequency) RF output, and is based on an oscillator using a YIG sphere as a resonator, and is tuned by changing the magnetic field applied to the YIG. The accuracy of the YIG tuned synthesizer is poor (typically 0.1% of the output frequency, xc2x118 MHz at 18 GHz). In addition, the tuning time is relatively slow (100 mS).
The direct digital synthesizer generates an RF output by sequencing through a series of addresses accessed from a RAM (Random Access Memory) that has previously been loaded with a mathematical sine function. The direct digital synthesizer produces a relatively fast RF output, but is limited in frequency resolution and bandwidth. The digitally tuned oscillator consists of a VCO that is digitally tuned, using a Programmable Read Only Memory (PROM) calibration table, to compensate for the nonlinear tuning characteristics of the VCO. To reduce the output frequency error over temperature, the VCO is often oven stabilized. While this produces a fast response ( less than 10 xcexcS), achievement of output frequency accuracies on the order of 1 MHz requires extensive calibration. Further, the oven stabilization leads to a long term frequency drift, requiring occasional recalibration. It is desired to provide a RF synthesizer that does not suffer the drawbacks of the prior art. More particularly, it is desired to provide a RF synthesizer having a high speed, and a broad band, and serves as an accurate signal source having long term frequency stability.
It is an object of the present invention to provide an RF synthesizer having a quick response, and a broad band, and provides an accurate signal source having long term frequency stability.
It is another object of the present invention to provide an RF synthesizer having a correlator that provides the RF synthesizer with long term stability.
It is still another object of the present invention to provide broad band frequency dividers that operatively cooperate with the correlator to provide long term frequency stability for the RF synthesizer.
The invention is directed to a digitally tuned synthesizer having a frequency measurement circuit, which includes a correlator that provides long term frequency stability to the synthesizer.
The digitally tuned synthesizer comprises: register means for receiving digital word having a predetermined number of bits; means interconnected to said register means for partitioning the predetermined number of bits of the digital word into coarse and fine control words; a first digital to analog converter connected to the coarse control word and providing an output representative of the coarse control word; at least one voltage controlled oscillator having first and second inputs with the first input connected to the output of the first digital to analog converter and providing a RF output representative of its first and second inputs. The digitally tuned synthesizer further comprises a frequency measurement circuit connected to the output of the at least one voltage controlled oscillator and providing an output representative thereof; a first adder having plus (+) and minus (xe2x88x92) inputs. The plus (+) input being connected to the fine control word. The minus (xe2x88x92) input being connected to the output of said frequency measuring circuit. The adder has an output representative of the difference between its inputs. The digitally tuned synthesizer further comprises a second digital to analog converter connected to the output of the first adder and providing an output representative thereof that is connected to the second input of the at least one voltage controlled oscillator.